1. Field of the Invention
The present invention relates to an optical element used in a technical field of optical communication, optical signal processing, and the like.
2. Description of the Related Art
Applying an electric field to a dielectric crystal or a ferroelectric crystal causes a change in refractive index. This phenomenon is called an electro-optic effect. This effect is utilized for developing various optical elements such as an optical modulator, a light deflector, and an optical switch.
Here, assuming that an electric field E is applied in parallel with TM-Mode of light propagating in a crystal. Changes in refractive indexes of a TE-Mode and a TM-Mode at this time are expressed as follows respectively.ΔnTE=−(½)nTE3r13E  (1)ΔnTM=−(½)nTM3r33E  (2)
nTE and nTM represent refractive indexes of the TE-Mode and the TM-Mode, and r13 and r33 represent electro-optic constants thereof, respectively. Variation amounts ΔnTE, ΔnTM of the refractive indexes are proportional to voltage intensity, electro-optic constant, and refractive index cubed. Using a material high in electro-optic effect to form an optical waveguide in an optical element, for example, a light deflector can result in a large-angle light deflection even with a low voltage. Therefore, the use of a material high in electro-optic constant and in refractive index is under consideration in various kinds of optical elements.
[Patent document 1] Japanese Patent Application Laid-open No. 2003-177262
[Patent document 2] Japanese Patent Application Laid-open No. 2001-117059
[Patent document 3] Japanese Patent Application Laid-open No. 2001-249311
[Patent document 4] Japanese Patent Application Laid-open No. Hei 6-67130
An electro-optic effect rc generally used as a characteristic of a so-called bulk electro-optic material or the like is represented by the following expression.rc=r33−(nTE/nTM)3r13  (3)
In a bulk (Pb1−yLa(3/2)y) (Zr1−xTix)O3, which is so-called PLZT (8/65/35), a large value rc=500 pm/V is recorded.
Characteristics of thin-film PLZT, for example, an epitaxial film of PLZT formed by epitaxial growth have been reported (Ishii Masatoshi et. al.: The 51st Spring Meeting 2004 (The Japan Society of Applied Physics and Related Societies, 29p-ZL-1)). This paper discusses characteristics that are obtained when PLZT is used as a material to form an epitaxial film with {010} crystal orientation on a major growth face. Here, it is reported that r13 and r33=30 pm/V to 40 pm/V, and rc=0 pm/V. Thus, while bulk PLZT exhibits a high electro-optic effect, thin-film PLZT is significantly inferior in electro-optic effect compared with a bulk PLZT.
In recent years, a more miniaturized and higher-performance optical element is being demanded, and in order to respond to this demand, the use of a thin-film electro-optic material is essential. However, at present, no suitable measure has been found for applying a thin-film electro-optic material to an optical element as described above, and it is now groped for.